Electron microscopic studies of pulmonary innervation will be focussed on to elucidate nerve-effector organ relationships. These relationships will be examined from different aspects. Histochemical techniques will be utilized to classify terminals according to their content of neurohumoral substances. Serial-section electron microscopy and three-dimensional reconstruction will be used to demonstrate the relationships of nerve terminals to ganglia, smooth muscle, mucosal cells, and other lung tissues. The three-dimensional information not only demonstrates hidden relationships which would be missed by regular techniques, it also lends itself to quantitative analyses of these terminal interactions. Sequential developmental studies of pulmonary innervation will be used to demonstrate not only chronological sequences but also the structural and histochemical changes which occur during development.